Strobe pulse generator



March 24, 1970 L. JASPER STROBE PULSE GENERATOR Original Filed July 1,1965 I NVENTOR f5 LE JASPER ATTORNEY LESLI United States Patent US. Cl.33324 3 Claims ABSTRACT OF THE DISCLOSURE A system including a pluralityof voltage sampling diode bridges each having separate D.C. inputs fornormally reverse biasing the diodes of each of the bridges off. Aseparate pulse generator is coupled to each bridge by a separatetransformer. Equal and opposite pulses are applied to the respectiveD.C. inputs of the bridges to forward bias the diodes on whenever therespective pulse generator is enabled by a logic level. Another pulsegenerator is coupled to all of the logically controlled pulse generatorsto operate the pulse generator that is logically enabled in synchronismwith a timing pulse.

This application is a division of application Ser. No. 562,190 filedJuly 1, 1966, now Patent No. 3,459,969, issued Aug. 5, 1969.

This invention relates generally to pulse generation, and moreparticularly relates to a system and subeombination thereof forproducing equal and opposite spike-shaped pulses.

There are a number of different applications for a pair of pulses havingequal and opposite amplitudes and shapes. One particularly importantapplication for such pulses is in the operation of voltage samplingprobes which utilize a rectifier bridge switch. Such a bridge switch hasan input point, an output point, and a pair of reverse bias points. Inorder to open the switch, potentials are applied to the reverse biaspoints of sufficient magnitude to insure that the diodes remain reversebiased under all input and output conditions. For most samplingoperations, it is desired to momentarily close the bridge switch so asto transfer a portion of the voltage present at the input at a precisepoint in time to the output. This is accomplished by applying equal andopposite pulses to the reverse bias points of such an amplitude andpolarity as to momentarily forward bias all of the diodes of the bridgeswitch. As a result, a portion of the difference between the voltage atthe input and the voltage at the output of the bridge switch istransferred to the output of the bridge switch.

The sampling resolution and accuracy of the bridge switch is highlydependent upon how narrow the pulses are and the extent to which thepulses are precisely equal and opposite. Thus, in order to have highresolution, it is important to have very short duration pulses.

In fully automated test systems such as that described in US.application Ser. No. 512,109, entitled Universal Electronic Test System,filed by Alford et al. on Dec. 7, 1965, and assigned to the assignee ofthe present invenvention, a plurality of different voltage sensingprobes each using a diode bridge switch are multiplexed so thatsuccessive measurements can be made on a plurality of diiferent leads ofa multilead component under test using the same measuring channel. Theseveral sampling probes are often operated in rapid sequence during aseries of tests and each sampling bridge must sample the waveformprecisely at the same point in time with respect to a timing pulse inorder to make accurate measurements.

3,5 03,017 Patented Mar. 24, 1970 ice An important object of thisinvention is to provide a system for generating equal and oppositevoltage pulses.

Another very important object of the invention is to provide a systemfor selectively producing a pair of equal and opposite pulses on any oneof a plurality of channels for selectively operating any one of aplurality of diode bridge switches.

A further object is to provide such a system wherein each of a pluralityof pulse producing channels is normally fully energized and therefore ina stable condition ready for immediate operation.

A further object is to provide such a system wherein a relatively lowspeed driver pulse is switched between channels rather than switchingeach of a plurality of channels producing a high speed pulse.

Another object is to provide a means for combining reverse biasingpotentials with the equal and opposite pulses without disturbing thebalance between the pulses.

Another object of the invention is to provide a pulse generator forproducing a relatively high voltage and high current pulse whichgenerates the minimum amount of noise in the remainder of the system asa result of electromagnetic radiation.

Yet another object is to provide a pulse generator for producing a highvoltage, high current pulse having a fast rise time suitable for drivinga snap-off diode from a low voltage, low current timing pulse.

These and other objects are accomplished in accordance with the presentinvention by means of a plurality of first pulse generator means forproducing equal and opposite pulses at the respective outputs thereof inresponse to a transition applied to the respective inputs thereof, asecond pulse generator means for producing said transition at the outputthereof in response to a timing pulse, and logic controllable switchingmeans interconnecting the output of the second pulse generator means andthe inputs of each of the first pulse generator means for selectivelyapplying said transition to any one of the inputs of the first pulsegenerator means.

In accordance with a more specific aspect of the invention, each of thefirst pulse generator means comprises a forward biased snap-oft diodeconnected in series with a transmission line stub transformer and anadjustable current source. A capacitor is connected between the cathodeof the snap-off diode and ground, and the transition is applied to thecathode of the diode to reverse bias the diode. A transformer means isconnected to the anode of the snap-off diode for producing equal andopposite pulses in response to the snap-off transition at the anode ofthe snap-01f diode.

In accordance with another aspect of the invention, the controllableswitching means comprises a capacitor and a switching diode connected inseries between the output of the second pulse generator means and theinput of each of the first generator means. Each of the switching diodesis connected to be fonward biased by the transition at the output of thesecond pulse generator means. Biasing means is provided for selectivelyreverse biasing each of the switching diodes to a level greater than theamplitude of the transition so that the transition will not propagatethrough the switching diode to the snap-off diode when the switchingdiode is reverse biased.

In accordance with a still more specific aspect of the invention, thesecond pulse generator means comprises first and second transistors eachof which is connected in a series circuit from a collector supplyvoltage to an emitter supply voltage comprised of a resistance, thecollecteremitter of the transistor, and a reverse biased diode. Acapacitor is connected between the collector of each transistor and theemitter supply voltage. The first transistor is turned on by common modeinput means connected to apply a voltage pulse to forward bias thebase-emitter junction of the first transistor so as to cause thetransistor to conduct and the emitter of the transistor to approach thevoltage stored on the capacitor connected between the collector of thetransistor and the emitter supply voltage. The voltage swing of theemitter of the first transistor is then applied by a second common modemeans to forward bias the base-emitter junction of the second transistorand thereby turn the second transistor on at a rapid rate to product asharp rising, high current pulse. The sharp rising, high current pulseis then applied through the switching means to reverse bias the selectedsnap-01f diode.

The novel features believed characteristics of this invention are setforth in the appended claims. The invention itself, however, as well asother objetcs and advantages thereof, may best be understood byreference to the following detailed description of an illustrativeembodiment, when read in conjuction with the accompanying drawing,wherein:

The figure is a schematic circuit diagram of a circuit constructed inaccordance with the present invention.

Referring now to the drawing, a system constructed in accordance withthis invention is indicated generally by the reference numeral 10. Thesystem is comprised generally of a power pulse generator indicatedgenerally by the reference numeral 12, a plurality of secondary pulsegenerators, only two of which are illustrated and indicated generally bythe reference numerals 14a and 14b, and a multiplex switching meansindicated generally by the reference numeral 18, which is used toselectively connect the output of the pulse generator 12 to the input ofone of the plurality of generators 14a and 14b.

The pulse generator 12 has input and output transistors and 22. Theinput transistor 20 is connected in a series circuit comprised of thecollector supply voltage terminal 24, which in one embodiment is +15.0v., resistor 26, the collector-emitter of transistor 20, and normallyreverse biased diode 28. A capacitor is connected between the collectorof transistor 20 and ground, which is the collector supply voltage.Similarly, the output transistor 22 is connected in a series circuitcomprised of the supply voltage terminal 24, resistor 32, thecollectoremitter of transistor 22, and normally reverse biased diode 34.A capacitor 36 is connected between the collector of transistor 22 andground. An input terminal 38 of the pulse generator is connected by theconductor of a ferrite cored length of coaxial cable 40 to the base ofinput transistor 20. The end of the shield of the cable 40 near theinput 38 is grounded, and the end of the shield adjacent the base oftransistor 20 is connected to the emitter of the transistor to provide acommon mode means for forward biasing the base-emitter junction of thetransistor 20 and maintaining conduction of the transistor even thoughthe emitter of transistor 20 raises to a high positive value. Theemitter of transistor 20 is connected through a variable resistor 42 anda second length of ferrite cored coaxial cable 44 to the base oftransistor 22. The input end of the shield of the coaxial cable 44 isgrounded, and the output end is connected through a small resistor 46 tothe emitter of transistor 22. The emitter of transistor 22 is consideredthe output of the pulse generator 12.

The output of the pulse generator 12 is connected to a bus 50 which inturn is connected by a capacitor 52 and switching diode 54 to the input56 of each of the pulse generators 14a and 14b. The switching diode 54is biased by a voltage divider network including a resistor 58 whichinterconnects a negative voltage supply terminal 60 and the anode ofswitching diode 54 and resistor 62 and the collector-emitter circuit oftransistor 64 which interconnect a positive voltage supply terminal 66and the anode of switching diode 54. The base of transistor 64 is biasedby a voltage divided including resistor 68 and transistor 70 andresistor 72 which is connected between the supply voltage terminal 66and ground. The base terminal 74 of the transistor 70 is the logic inputfor controlling the respective switching diodes 54 as will hereafter bedescribed in greater detail.

Each of the pulse generators 14a is comprised of a snap-off diode whichis normally forward biased through a series circuit from ground throughfirst the shield and then the conductor of a shorted stub transformer82, the diode 80, fixed resistor 84 and variable resistor 86 to thenegative voltage supply terminal 60. A capacitor 88 is connected betweenthe cathode of the diode 80 and ground. The anode of the diode 80 isconnected to the input 90 of a first Balun transformer 91. The Baluntransformer 91 is comprised of a first length of coaxial transmissionline 92 having a ferrite core and a second matched length of coaxialtransmission line 89. The input 90 is the input end of the shield of thecoaxial transmission line 92. The conductor of the transmission line 92is common with the conductor of the transmission line 89. The input endof the shield of the transmission line 89 is connected to ground, andthe output ends of the shields of both transmission lines 92 and 89 arecommon and connected toground.

A second Balun transformer indicated generally by the reference numeral93 is comprised of a first length of ferrite cored coaxial transmissionline 94 and a second length of ferrite cored coaxial transmission line96. The to outputs of the Balun transformer 91 are connected to theinput ends of the shields of the transmission lines 94 and 96. The outerends of the shields are connected to ground. The input ends of theconductors of the transmission lines 94 and 96 are interconnected by acapacitor 98, and are connected to reverse bias supply terminals 100 and102. The output ends of the conductors are connected to the windings 104and 106 of an isolation transformer and finally to reverse bias inputs108 and 110 of a rectifier bridge switch indicated generally by thereference numeral 112. The rectifier bridge switch 112 is comprised offour diodes connected as illustrated and has an input 114 and an output116.

OPERATION Assume that all power supplies are active and that transistors20 and 22 are turned off so that capacitors 30 and 36 will each becharged to approximately +15.0 v. The snap-off diodes 80 will be forwardbiased to a level sufficient for the diodes to operate in the snap-0Emode, and the capacitors 88 will be charged to a voltage equal to theforward voltage drop across the respective diodes 80. Reverse biaspotentials will be applied at the reverse bias supply terminals 100 and102 so as to reverse bias all of the diodes of the rectifier bridgeswitches 112. Only one of the control terminals 74 is at a logic 1 levelof +4.0 v., and the remaining terminals 74 are at a logic 0 level ofground potential. For purposes of this description, assume that controlinput 74 associated with the pulse generator 14a is at a logic 1 levelof +4.0 v., and that all other control inputs 74 are at a logic 0 level.

When a positive timing pulse having an amplitude of +1.0 v. and a widthof 10 nanoseconds is applied to input 38, the pulse is passed throughthe transformer 40 and applied from base to emitter of the transistor20, thus forward biasing the base-emitter junction and tuming thetransistor 20 on. The purpose of the transformer 40 is to provide acommon mode connection so that the base-emitter junction can continue tobe forward biased by +1.0 v. even though the emitter of the transistor20 almost immediately approaches the voltage across the capacitor 30,which is approximately +15.0 v. This provides a +l5.0 v. pulse at theoutput of the first stage, i.e., the emitter of transistor 20, from a+1.0 v. input pulse. The diode 28 is provided to clamp off the reverseswing from the transformer 40 in order to prevent damage to thetransistor 20. The +l5.0 v. swing of the emitter of the transistor 20 ispassed through the variable resistor 42 and transformer 44, which alsoprovides common mode operation, and is applied to forward bias thebase-emitter junction of transistor 22. The resistor 42 is made variablein order to adjust the current to the base of 'transistor 22. Theresistor 46 is provided to prevent accidentally overdriving thetransistor 22, should the resistor 42 be adjusted to a very low value.The +l5.0 v. pulse produced by the swing of the emitter of transistor 20provides sufficient current to rapidly drive transistor 22 intosaturation, thus producing a high current pulse having a fast rise timewhich is applied to the output bus 50. The diode 34 again preventsdamage to the transistor 22 during the reverse swing of the trans former44.

It will be recalled that it was assumed that the logic control input 74for the pulse generator 14a was at a logic 1 level of +4.0 v. Thustransistor 70 is turned on which in turn turns transistor 64 on, Sinceresistors 62 and 58 are substantially equal, the anode of switchingdiode 54 is approximately at ground potential. The positive pulse ofcurrent applied to the bus 50 can then pass through the capacitor 52 andthrough the forward biased switching diode 54 to charge the capacitor 88and reverse bias the snap-off diode 80 as will presently be described.However, since the input 74 associated with the other pulse generators14b, etc., are at a logic 0 level of ground potential, the transistors70 and 64 of the other channels are turned off and the anode of theswitching diode 54 is at approximately l5.0 v. Therefore, when the +15.0v. pulse is applied to the capacitors 52 of the channels 1411, etc'.,the anode of the corresponding switching diode 54 is not madesufficiently positive to cause the diode to conduct and no positivepulse is applied to reverse bias the respective snap-off diode 80.

As previously mentioned, the snap-off diode 80 is continually forwardbiased through the series circuit extending from ground through theshield and then the conductor of the shorted stub coaxial transformer82, the diode 80, and resistors 84 and 86 to the negative voltage supplyterminal 60. Thus the forward biased current through the diode 80 isconstant whether the particular pulse generator 14 is connected ordisconnected from the pulse generator 12 by the switching means 18. Asmentioned, the forward voltage drop through the snap-off diode 80 isimpressed across the capacitor 88. When the positive pulse from thegenerator 12 passes through the diode 54 and hits the snap-off diode 80and capacitor 88, the diode 80 momentarily conducts in the reversedirection as all carriers are swept from the junction, the stopsconducting very abruptly. The capacitor 88 prevents the reverse currentfrom changing in the stub transformer 82 at a rate which would producean undesirable voltage change. This abrupt cessation in the currentthrough the stub transformer 82 generates a negative pulse which isapplied to input 90 of the Balun transformer 91. The point at which thepulse occurs with respect to the incoming positive pulse and thereforewith respect to the timing pulse applied to input 38, is de termined bythe setting of adjustable resistor 86, and the width of the negativepulse is twice the time length of the coaxial stub transformer 82.

The negative pulse is applied to the input 90 of the first Baluntransformer 91 which produces equal and opposite pulses in theconductors of the coaxial cables 92 and 89, the pulse on the conductorof cable 92 being positive and that from cable 89 being negative. Thepositive and negative pulses are then applied to the shields of coaxialcables 94 and 96, respectively, of the second Balun transformer 93. Thepositive pulse applied to the shield of cable 94 produces a negativepulse in the conductor of the coaxial line 94, and the negative pulseapplied to the shield of coaxial line 96 produces a positive pulse inthe conductor. A positive bias voltage is applied to terminal 100 and asubstantially equal negative bias voltage is applied to the terminal102. These DC. bias voltages are fed through the conductors of thecables 94 and 96 and the windings 104 and 106 to the reverse bias points108 and 110. It is important to note that the conductors of coaxialcables 94 and 96 which carry the DC. bias to the bias points 108 and 110of the diode bridge 112 are electrically isolated from the conductors ofcoaxial cables 92 and 89 except by inductive coupling, and are isolatedfrom one another by the capacitor 98. The combination of the twotransformer stages and the capacitor 98 permits the introduction of thereverse bias potential at a neutral point resulting in a minimumdisturbance of the balanced pulses. The negative pulse induced in theconductor of coaxial cable 94 and the positive pulse induced in theconductor of coaxial cable 96 then propagate through the windings 104and 106 of the isolation transformer and overcome the reverse D.C. biasapplied to reverse bias points 108 and 110 so as to momentarily forwardbias the diodes of the bridge switch 112. During the period of time thatthe diodes are forward biased, a portion of the algebraic differencebetween the voltage at the input 114 and the voltage at the output 116,the portion depending upon the sampling efficiency of the bridge switch,will be transferredfrom the input to the output in a manner known in theart.

From the above detailed description of a preferred embodiment of theinvention, it will be noted that a relatively slow speed pulse isgenerated by the pulse generator 12 and this pulse is multiplexed by theswitching means 18 as opposed to the multiplexing of the high frequencypulses produced by the several pulse generation channels 14. It willalso be noted that each of the snap-off diodes is always conductingcurrent in the forward direction and therefore is continually maintainedat a substantially constant operating temperature regardless of the dutycycle of the particular channel. This minimizes variations in theamplitudes of the pulses and variations in the time at which the pulsesare produced as a result of imbalanced and changing duty cycles amongthe various channels. Further, since the transistors of the pulsegenerator 12 operate in the saturation mode, rather than the avalanchemode as is often the case in pulse generators having a high currentrequirement, the noise generated in the adjacent system due to radiantenergy is substantially reduced. Yet the pulse generator 12 produces apulse having a sufiiciently fast rise time that the reverse currentthrough the snap-off diode 80 substantially stabilizes before the diodeabruptly stops conducting. As a result, the amplitude and timing of theequal and opposite pulses generated remain substantially constant.

Although a preferred embodiment of the invention has been described indetail, it is to be understood that various changes, substitutions andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:

1. In a system for producing equal and opposite pulses superimposed uponbiasing potentials, the combination of a pair of transmission lines eachhaving a conductor and a shield each of which has an input and an outputend, a capacitor interconnecting the input ends of the conductor, meansgrounding the output ends of the shields, the input ends of the shieldsbeing the inputs for equal and opposite pulses, the input ends of theconductors being the inputs for the biasing potentials, and the outputends of the conductors being the output for the combined pulses andbiasing potentials.

2. The combination defined in claim 1 further char acterized by secondcircuit means connected to the input ends of shields for producingsubstantially equal and opposite pulses at the respective input endsfrom a single timing pulse applied to the input thereof and thirdcircuit means connected to the input of the second circuit means forapplying a timing pulse.

7 8 3. The combination defined in claim 2 further char- References Citedacterized by a transformer comprised of a third and fourth UNITED STATESPATENTS transmission line each having a conductor and a shield each ofwhich has an input end and an output end, the 2,149,387 3/1939 Brown 8input end of one shield being the input of the transformer, the inputend of the other shield being grounded, the 5 HERMAN KARL SAALBACHPnmary Exammer output ends of the shields being grounded, the input endsMARVIN NUSSBAUM, Assistant Examiner of the conductors beinginterconnected, and the output ends of the conductors being connected tothe input ends US. Cl. X.R.

of the shields of said pair of transmission lines. 10 3331, 97

